Lithium metal phosphates with olivine structure have emerged as promising cathode materials in secondary lithium-ion batteries. The advantages of the lithium metal phosphates, compared with other lithium compounds based on spinels or layered oxides, are their environmental benignity and their safety properties during battery handling and operation.
The poor electrochemical performance of pure lithium iron phosphate was improved by coating the particles with carbon.
The disadvantage of lithium iron phosphate is in the lower Fe3+/Fe2+ redox couple having a flat potential curve at 3.4 V vs. Li/Li+ as opposed to conventional oxide chemistries having an average potential of 3.7 V vs. Li/Li+. In order to improve the electrochemical properties of the lithium metal phosphate in terms of average voltage, it is tempting to substitute fully the iron by manganese in the lithium iron phosphate crystal structure because the Mn3+/Mn2+ redox couple causes a flat potential curve at 4.1 V vs. Li/Li+. However, in most of the practical cases, the full capacity at 4.1 V is not achieved for pure lithium manganese phosphate without iron coexistence.
Melting processes, hydrothermal processes and solid-state processes are the most common synthesis routes for the preparation of lithium metal phosphates.
WO 2005062404 A1 discloses a melting process for the preparation of lithium metal phosphate by melting the starting materials comprising a metal compound, a lithium compound and a phosphate compound at a temperature of about 1000° C. under non-reactive or partially reducing atmosphere.
EP 1682446 B1 discloses the preparation of lithium metal phosphate through the reaction of a Li-source, at least one M-source (M can be Fe, Mn, Co, Ni) and at least one PO4-source under hydrothermal conditions at a temperature between 100° and 250° C. and at a pressure from 1 to 40 bar. The addition of an electrically conducting material before heat treating is also described.
U.S. Pat. No. 5,910,382 C1 and U.S. Pat. No. 6,514,640 C1 disclose a solid state synthesis route for the preparation of LiMPO4. The starting materials containing a Li-source, a M-source (M can be Fe, Mn, Co, Ni) and a PO4-source are mixed, calcined between 300° C. and 350° C. and then heated to about 800° C. in argon.
EP 2458666 A1 describes a nano particulate LiMPO4 as cathode material where M is selected from at least one metal of the group of Mn, Fe, Co and Ni. The nano particulate LiMPO4 is obtained from LiMPO4 prepared by well known methods, e.g. a solid state synthesis route, or from precursor materials of LiMPO4. The disclosed process comprises mixing LiMPO4 or the precursor materials thereof with a carbon precursor at ambient temperature, adding a stabilizing agent, wet milling the mixture, drying and calcining the obtained mixture.